This invention relates to motion-tracking.
Inertial tracking with automatic drift correction has been demonstrated to be a successful technique for tracking objects, such as limbs, cameras, input devices, or head mounted displays (HMDs), offering low jitter, fast response, increased range, and reduced problems due to interference or line-of-sight occlusion. Inertial trackers have been successfully applied to a wide range of HMD applications including virtual environment (VE) training, virtual prototyping, interactive visualization and design, VR gaming, and even fixed-base vehicle simulation. Within this gamut of applications, inertial trackers have gained widespread acceptance as a high-performance, robust and cost-effective alternatives to magnetic, optical and acoustic tracking systems. InterSense of Burlington, Mass., has pioneered the commercial development of motion tracking systems using miniature MEMS-based inertial sensors, and now offers a broad product line of inertial hybrid trackers.
Until now, inertial trackers have not been used in applications that require tracking motion relative to a moving platform instead of relative to the earth. This includes such important applications as motion-base driving and flight simulators, conventional VE systems deployed on board ships, and a range of live vehicular applications such as driver""s or pilot""s vision enhancement, helmet-mounted cueing systems, and advanced human-machine interfaces to improve pilots"" situational awareness and control capability. People wishing to use inertial trackers in these types of applications have been realized that standard inertial tracking systems such as the InterSense IS-300, 600 or 900 will not function correctly if operated on a moving platform such as a motion-base simulator or vehicle. The inertial sensors would measure head motion relative to the ground, while the drift-correcting range sensors would measure head pose relative to the vehicle platform in which the reference receivers are mounted. While the vehicle is turning or accelerating, the Kalman filter would attempt to fuse inconsistent data and produce unpredictable results.
The invention makes it possible to use inertial head-tracking systems on-board moving platforms by computing the motion of a xe2x80x9ctrackingxe2x80x9d Inertial Measurement Unit (IMU) mounted on the object being tacked relative to a xe2x80x9creferencexe2x80x9d IMU rigidly attached to the moving platform. The advantages of the invention apply not only to inertial tracker with drift correction using ultrasonic ranging sensors, but also to hybrid inertial trackers involving optical, magnetic, or RF drift correction as well.
In general, in one aspect, the invention features a system for tracking the motion of an object relative to a moving reference frame. The system includes a first inertial sensor mounted on the tracked object; a second inertial sensor mounted on the moving reference frame; and an element coupled to the first and second inertial sensors and configured to determine an orientation of the object relative to the moving reference frame based on the signals from the first and second inertial sensors.
Implementations of the invention may include one or more of the following features. The first and second inertial sensors may include three angular inertial sensors selected from the set of angular accelerometers, angular rate sensors, and angular position gyroscopes. The angular inertial sensors may include angular rate sensors, and the orientation of the object relative to the moving reference frame may be determined by integrating a relative angular rate signal determined from the angular rate signals measured by the first and second inertial sensors. A non-inertial measuring subsystem may make independent measurements related to the orientation of the object relative to the moving reference frame, and use them for correcting any drift that may occur in the inertial orientation integration. The non-inertial measuring subsystem may be selected from the set of optical, acoustic, magnetic, RF, or electromagnetic technologies.
The determination of relative orientation may be done by computing the orientation of the object with respect to a fixed inertial reference frame using the signals from the first inertial sensor, the orientation of the moving reference frame with respect to the same fixed inertial reference frame using the signals from the second inertial sensor, and the relative orientation based on the two orientations.
A drift corrector may be used to correct inertial drift in the determined orientation of the object with respect to the inertial reference frame or of the moving reference frame with respect to the inertial reference frame. The drift corrector may include sensors for determining tilt with respect to earth""s gravitational field, heading with respect to earth""s magnetic field. A drift corrector may be used for correcting inertial drift in the determined orientation of the object with respect to the moving reference frame by using non-inertial sensors to independently measure the relative orientation.
The first and second inertial sensors may each include three linear accelerometers. An element may be included for calculating the position of the object relative to the moving reference frame. The calculating element may double-integrate a relative linear acceleration signal computed from the linear accelerometer signals measured by the first and second inertial sensors. The calculation of the relative linear acceleration signal may include compensation for tangential, Coriolis and centripetal acceleration effects caused by the angular velocity and angular acceleration of the moving reference frame. The compensation terms may be calculated using the angular velocity or angular acceleration of the moving reference frame measured by the second inertial sensor. In some implementations no compensation for the effect of gravity on the accelerometers is made. The calculation of the position of the object relative to the moving reference frame may include computing the position of the object with respect to a fixed inertial reference frame using the signals from the first inertial sensor, the position of the moving reference frame with respect to the same fixed inertial reference frame using the signals from the second inertial sensor, and the relative position based on the two individual positions. A drift corrector may correct for inertial drift in the determined position of the object with respect to the inertial reference frame or of the moving reference frame with respect to the inertial reference frame. The drift corrector may include sensors for measuring position of both the object and the moving reference frame with respect to landmarks fixed in common inertial reference frame. The moving reference frame may be associated with a vehicle, and the second inertial sensor may include a pre-existing inertial measurement unit on a vehicle that was installed for the purpose of navigation. The first and second inertial sensors may each include at least six linear accelerometers and associated processors to extract three angular inertial signals and three linear accelerations.
Other advantages and features will become apparent from the following description and from the claims.